With the remarkable recent development of the electronics industry, various electronic components have been made more and more sophisticated and precise.
For instance, a glazed ceramic base can be prepared by forming a glass layer on an alumina or beryllia base, and can be used for resistor elements, integrated circuits utilizing highly precise circuits, thermal heads of heat-sensitive recording apparatuses, etc.
With respect to the use of the glazed base as a thermal head, there is a thick film system and a thin film system. The thick film system enables the wiring of 6 dots/mm or more, and the electric potential to be applied can be reduced to 1/2 that which would otherwise be required by inserting a heat regenerative glass layer; thus the thermal efficiency is improved. The thin film system enables the wiring of 8 dots/mm or more. Therefore, it would be very desirable to obtain an alkali-free and PbO-free glaze having good flatness of the glaze surface, good heat resistance and good corrosion resistance to HF+HNO.sub.3.
Alkali and PbO are extremely efficient for improving good flatness. However, in case that the alkali-containing glazed base is used as the thermal heads, alkali is gradually diffused toward a portion where the electric potential decreases due to an applied voltage to make a protuberance on the surface thereof and in a short time the surface is destroyed. On the other hand, in case that the PbO-containing glazed base is used as the thermal heads, a Pb.sup.2+ ion is reduced to a Pb.sup.0 metal to decrease an electric insulation of the glazed base while in a step for fusing together a wiring pattern on the surface, the glazed base is exposed with a reducing atmosphere (when a base metal such as Cu, or etc., which is available in a low price is used as the wiring pattern, the reducing atmosphere is usually selected in order to prevent oxidation thereof).
Further, it is very desirable for the glaze to have a low thermal conductivity, for example, a value of K=0.002 Cal/cm.sec..degree.C. A thermal conductivity of such a low value enables the use of a thin glaze layer, and hence protuberances and projections of the glaze are reduced so much that a flat-surface glazed ceramic base can be formed.